When you put something such as "n/a" in a formula Monarch treats it as a character field. However all is not lost.
This will give you a character field with the n/a showing. Then you can use the Val function to change that field back to numeric in any other calculated field. The n/a will change to a zero when you use the Val function.
So you should end up with a field showing the n/a where you need it and other fields being calculated from it.
I am wondering it you want to make the result of all fields less than 0 a null field (in effect) or for them to hold the REAL value but display as "n/a".
For example you could could use a formula like
IF(YOURFIELD<0, YOURFIELD/0, YOURFIELD*.03)
This would give a NULL result if the value was less than 0 and then you could set your options to display null as n/a.
However further calculations with the NULL value would not be possible unless a zero result was acceptable - you would need to perform the calculation from the ORIGINAL field - i.e. YOURFIELD.
Does this help?